Various versions of stable thermostatically controlled quartz crystal oscillators are known to the prior art (see, for example U.S. Pat. No. 4,985,687 and French Patent No. 2,660,499, 2). To achieve temperature stability up to .+-.1.times.10.sup.-8 prior art frequency oscillators usually employed a one-stepped thermostatic circuit. This allows a most simple, economical and compact design of oscillator to be achieved. To achieve stability above .+-.1.times.10.sup.-9 a two-stepped thermostatic circuit is used. A higher precision is achieved in this case at the cost of more complex and cumbersome design (see. Ingberman M. I., Fromberg E. M, Graboi L. P., Thermostatic Methods in Communication Technology., M., Sviaz, 1979, page 17, and also Godkov V. D., Gromov S. S., Nikitin N. V., Thermostatic Methods and Wireless Appliances, Moscow, Goskomizdat, 1979, page 46).
RU 2081506 describes a crystal oscillator with a one-stepped thermostatic circuit, which circuit contains a circuit board with elements of the oscillator system mounted thereon. The circuit board is installed in a hermetically sealed outer housing. All temperature controlled elements of the system, including a thermostat with built-in crystal resonator, heating elements, a temperature detector and a thermostatic regulator with a thermosensitive bridge circuit are located in the central part of the circuit board. The central part of the circuit.board is separated from its peripheral part by means of through cuts, both parts being connected at the ends of the cuts by narrow bridging strips.
This oscillator has obvious advantages, such as a simple design, small size and low cost, but it does not allow temperature stability of .+-.1.times.10.sup.-9 to be achieved. This limitation is explained by insufficient uniformity of temperature distribution and insufficient precision in maintaining a constant temperature.